1. Field of the Invention
The present invention relates to a displacement information measurement apparatus and an optical device and, more particularly, to a displacement information measurement apparatus for optically measuring the displacement information of a moving object in a non-contact manner and an optical device suitable for the apparatus.
2. Related Background Art
Conventionally, as an apparatus for precisely measuring the displacement information of an object in a non-contact manner, a laser Doppler velocimeter (LDV), a laser encoder, or the like is used. The laser Doppler velocimeter measures the moving velocity of a moving object or fluid by utilizing an effect (Doppler effect) in that when a laser beam is irradiated onto a moving object or fluid, the frequency of the laser beam scattered by the moving object or fluid shifts in proportion to the moving velocity.
FIG. 1 shows an example of the conventional laser Doppler velocimeter. The laser Doppler velocimeter shown in FIG. 1 comprises a laser 1, a collimator lens 2 for obtaining a collimated light beam 3, a beam splitter 4, mirrors 6a, 6b, 6c, and 6d, a focusing lens 8, and a photodetector 9. An object 7 to be measured is moving in the direction of an arrow at a velocity V.
A laser beam emitted by the laser 1 is converted into the collimated light beam 3 by the collimator lens 2, and the beam 3 is split into two light beams 5a and 5b by the beam splitter 4. These light beams 5a and 5b are respectively reflected by the mirrors 6a and 6c, and the mirrors 6b and 6d, and are irradiated onto the object 7 to be measured, which is moving at the velocity V, at an incident angle .theta.. Light scattered by the object or fluid is detected by the photodetector 9 via the focusing lens 8. The frequencies of the two scattered light beams are subjected to Doppler shifts of +f and -f in proportion to the moving velocity V. Let .lambda. be the wavelength of the laser beam. Then, f can be expressed by equation (1) below: EQU f=V.multidot.sin (.theta.)/.lambda. (1)
The scattered light beams which have been subjected to the Doppler shifts of +f and -f interfere with each other, and cause a change in density pattern on the light-receiving surface of the photodetector 9. The frequency, F, of the interference light is given by equation (2) below: EQU F=2.multidot.f=2.multidot.V.multidot.sin (.theta.)/.lambda.(2)
From equation (2), the velocity V of the object 7 to be measured can be obtained by measuring the frequency F (to be referred to as the Doppler frequency hereinafter) of the photodetector 9.
In the above-mentioned conventional laser Doppler velocimeter, as can be seen from equation (2), the Doppler frequency F is inversely proportional to the wavelength .lambda. of the laser beam, and hence, the laser Doppler velocimeter must use a laser light source with a stable wavelength. As a laser light source which is capable of continuous laser oscillation and has a 10 stable wavelength, a gas laser such as an He-Ne laser is popularly used. However, such a gas laser requires a large laser oscillator and a high voltage in a power supply, resulting in a large, expensive apparatus. On the other hand, a laser diode (or a semiconductor laser) used in compact disk drives, video disk drives, optical fiber communications, or the like has temperature dependence although it is very small and can be easily driven.
FIG. 2 shows an example of the typical temperature dependence of a laser diode (quoted from '87 Mitsubishi Semiconductor Data Book; Volume of Optical Semiconductor Elements). A continuous change in wavelength is mainly caused by a change in refractive index of the active layer of the laser diode due to a change in temperature, and is 0.05 to 0.06 nm/.degree.C. On the other hand, a discontinuous change in wavelength is called longitudinal mode hopping, and is 0.2 to 0.3 nm/.degree.C.
In order to stabilize the wavelength, in general, a method of controlling the laser diode at a given temperature is adopted. In this method, temperature control members such as a heater, radiator, temperature sensor, and the like must be attached to the laser diode to have a small heat resistance, and temperature control must be performed precisely. As a result, the laser Doppler velocimeter becomes relatively large in size and its cost increases- In addition, instability due to the above-mentioned longitudinal mode hopping cannot be perfectly removed.
As a laser Doppler velocimeter which can solve the above-mentioned problems, the following system has been proposed. That is, a laser beam as a light source is incident on a diffraction grating, two diffracted light beams of the +nth and -nth orders, other than the 0th order, of the diffracted light obtained from the diffraction grating are irradiated onto a moving object or fluid at the same crossing angle as the angle defined by these two light beams, and scattered light from the moving object or fluid is detected by a photodetector.
FIG. 3 shows an example of diffraction when a laser beam I is incident on a transmission type diffraction grating 10 with a grating pitch d in a direction perpendicular to the alignment direction, t, of grating lines. The diffraction angle, .theta.0, is given by: EQU sin .theta.0=m.lambda./d
where m is the order (0, 1, 2, . . . ) of diffraction, and .lambda. is the wavelength of light.
Of these light beams, .+-.nth-order light beams other than the 0th-order light are expressed by: EQU sin .theta.0=.+-.n.lambda./d (3)
for n =1, 2, . . .
FIG. 4 shows a case wherein the two light beam, i.e., the .+-.nth-order light beams are irradiated onto the object 7 to be measured via the parallel mirrors 6a and 6b to have an incident angle .theta.0. From equations (2) and (3), the Doppler frequency F of the photodetector 9 is given by: EQU F=2V sin .theta.0/.lambda.=2nV/d (4)
The frequency F does not depend on the wavelength .lambda. of the laser beam I, is inversely proportional to the grating pitch d of a diffraction grating 20, and is proportional to the velocity of the object 7 to be measured. Since the grating pitch d can be sufficiently stabilized, the Doppler frequency F is proportional to only the velocity of the object 7 to be measured. The same applies to a case wherein the diffraction grating 20 comprises a reflection type diffraction grating.
In general, when highly coherent light such as a laser beam is irradiated onto an object, scattered light is randomly phase-modulated by a minute surface structure on the object, and forms a dot pattern, a so-called speckle pattern, on the observation surface. In the laser Doppler velocimeter, when an object or fluid moves, a change in density pattern due to the Doppler shift on the detection surface of the photodetector is modulated by an irregular change in density pattern due to the flow of the speckle pattern, and the output signal from the photodetector is also modulated by a change in transmittance (or reflectance) of an object to be measured.
In the above-mentioned LDV, since the frequency of change in density pattern due to the flow of the speckle pattern and the frequency of change in transmittance (or reflectance) of the object to be measured are lower than the Doppler frequency given by equation (2), the output from the photodiode is supplied to a high-pass filter to remove the low-frequency components, thereby extracting only a Doppler signal. However, if the velocity of the object to be measured is low and therefor the Doppler frequency is low, the frequency difference from the low-frequency variation components becomes small, and a high-pass filter cannot be used. As a result, the measurement of the velocity of the object to be measured may be disabled. In addition, the velocity direction cannot be detected in principle.
As a technique for attaining a measurement including that of the velocity direction from a still state, a method (frequency shifter) of setting a frequency difference between two light beams before the two light beams are irradiated onto an object to be measured is known.
In general, an acoustooptic element is used as the frequency shifter. In this case, since the incident angle of light beams on the acoustooptic element must be set to the Bragg diffraction angle, the acoustooptic element cannot be inserted in an optical system in which the optical path changes depending on the wavelength, as shown in FIG. 4.
FIG. 5 shows an example of a laser Doppler velocimeter which uses a frequency shifter consisting of an electrooptic crystal in place of the acoustooptic element.
In the electrooptic crystal, the refractive index of the medium changes depending on the electric field to be applied. For example, trigonal systems 3m such as LiNbO.sub.3, LiTaO.sub.3, and the like, and tetragonal systems 42m such as (NH.sub.4)H.sub.2 PO.sub.4 (ADP), KH.sub.2 PO.sub.4 (KDP), and the like are known. The following explanation will be given taking LiNbO.sub.3 as an example.
The index ellipsoid of LiNbO.sub.3 (3m) is given by: EQU (1/N.sub.o.sup.2 -.gamma..sub.22 E.sub.2 +.gamma..sub.13 E.sub.3)X.sup.2 +(1/N.sub.o.sup.2 +.gamma..sub.22 E.sub.2 +.gamma..sub.13 E.sub.3)Y.sup.2 +(1/N.sub.e.sup.2 +.gamma..sub.33 E.sub.3)Z.sup.2 -2.gamma..sub.22 E.sub.1 XY+2.gamma..sub.51 E.sub.2 YZ+2.gamma..sub.51 E.sub.1 ZX=1(5)
where .gamma. (with a suffix) is a Pockels constant, and N.sub.o and N.sub.e are respectively the refractive indices of ordinary and extraordinary rays.
This apparatus adopts an arrangement for applying a voltage to this electrooptic crystal. If the rate of change in voltage V per unit time is set to be constant, light transmitted through LiNbO.sub.3 has a constant rate of change in phase amount per unit time. In other words, the crystal serves as a frequency shifter. However, when the voltage is changed at a constant rate, the voltage increases infinitely. For this reason, in practice, a sawtooth wave (serrodyne) driving operation is performed, as shown in FIG. 6. The driving operation is performed by a value with which the voltage amplitude corresponds to an optical phase of 2.pi., so that the optical phase does not become discontinuous in a return portion. If the serrodyne frequency is represented by fR, the light beam I is frequency-shifted by fR.
The apparatus shown in FIG. 5 utilizes the above-mentioned principle, and has been described by Foord et al., (Appl. Phys., Vol. 7, 1974, L36-L39). A laser beam is split into two light beams by the beam splitter 4, and these light beams are transmitted through an electrooptic crystal 10. The two light beams which have been frequency-shifted by the serrodyne driving operation are deflected by a lens 15, and cross in a convergent state. This arrangement is normally utilized as a current meter, and attains measurements including that of the velocity direction from a still state. The Doppler frequency is given using the frequency difference fR between the two light beams by the following equation: EQU F=2.multidot.V.multidot.sin (.theta.)/.lambda.+fR (6)
Therefore, even when the velocity V of the object 7 to be measured is low, if fR is set to be an appropriate value, a sufficient frequency difference from the low-frequency component caused by the flow of the speckle pattern or the change in transmittance (or reflectance) of the object to be measured can be assured, and only a Doppler signal is extracted by electrically removing the low-frequency components, thus allowing velocity detection.
Conventionally, the laser beam is perpendicularly incident on the electrooptic crystal, as shown in FIG. 5, and, to insert an electrooptic crystal in an optical system, it is a common practice to adopt a structure in which a collimated light beam is perpendicularly incident and the optical path is immovable. In the conventional arrangement, when the incident angle of the light beams varies, then a relationship between the electric field to the electrooptic crystal, and the change in refractive index also changes. As a result, even when the voltage amplitude of the serrodyne driving operation is set to be constant, the change in optical phase deviates from 2.pi..
However, if the optical path is immovable, as can be seen from equation (6), when the laser wavelength varies, the Doppler signal to be detected varies, thus disturbing high-precision measurement.